1. Technical Field
The invention is related to electro-optical sensors which scan a field of view, and in particular to electro-optical sensors which view a scene through a wide-field-of-view objective lens corresponding to a focal plane larger than the sensor array.
2. Background Art
Electro-optical (E-O) sensors, particularly those useful for sensing infrared images, typically contain either a linear array on the order of 128 image-sensing elements (photosensitive diodes) or a planar array of 128 rows and 128 columns of such image-sensing elements. Hereafter, such image-sensing elements will be referred to herein as "pixels". The problem with such sensors is their small size, particularly whenever they are used in conjunction with a viewing lens having a very wide field of view (FOV). The image plane of such lenses corresponding to the very wide field of view is larger than the sensor. In order to capture the entire FOV of such a lens, a mechanical scanner (a gimballed mirror) is employed to compensate for the small size of the sensor. The scanner causes the sensor to view, in succession, small sub-sections of the entire image until the whole image has been captured and stored. One persistent problem with such systems is that the scanner's mechanical components (the mirror gimbal transmission and motor) is the least reliable component of the entire system.
It would not be feasible to discard the mechanical scanning of the system. To do so would require that the entire wide-FOV image be focussed onto the E-O sensor array, which would greatly reduce the resolution of the image recorded by the sensor. Otherwise, a major portion of the wide-FOV image would fall outside the sensor array and be lost. Alternatively, a very large E-O sensor array could be employed, but such large arrays are prohibitively expensive for purchase in large quantities. Thus, for inexpensive E-O sensor arrays (on the order of 128-by-128 pixels), mechanical scanning seems to be an unavoidable necessity.
Wide FOV lenses and optical field flatteners therefor are known in the art, as disclosed in U.S. Pat. Nos. 3,961,844 and 4,772,107. Spatial light modulators (SLM's) are used in the art by having each pixel of the SLM control a respective pixel of an image. This technique is disclosed in U.S. Pat. No. 4,948,258 for ranging systems and in U.S. Pat. Nos. 5,132,813 and 5,056,039 for optical interconnects for processors or computing systems.
What is needed is a way of using a relatively small E-O sensor to capture the entire field of view of a wide-field-of-view lens without requiring a mechanical scanner such as a gimballed mirror.